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United States Patent |
5,350,923
|
Bassignana
,   et al.
|
September 27, 1994
|
Apparatus for use with analytical measuring instruments using
electromagnetic radiation analysis methods
Abstract
A method and apparatus for use in performing non-contact analytical
evaluation of a semiconductor wafer, which needs to be kept clean, to be
performed outside of clean room facilities. The apparatus maintains a
clean environment surrounding the semiconductor wafer and a portion of the
apparatus is substantially transparent to a probe beam of electromagnetic
radiation such as X-rays and visible light. The invention substantially
overcomes the expenses associated with locating analytical test equipment
for testing semi-conductor wafers within clean room facilities.
Inventors:
|
Bassignana; Isabella C. (Ottawa, CA);
Kovats; Tibor F. I. (Ottawa, CA)
|
Assignee:
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Northern Telecom Limited (Montreal, CA)
|
Appl. No.:
|
996411 |
Filed:
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December 23, 1992 |
Current U.S. Class: |
250/453.11; 356/244; 378/79; 378/161 |
Intern'l Class: |
H01J 037/20 |
Field of Search: |
250/453.11
378/161,70,79,80
356/244
|
References Cited
U.S. Patent Documents
3973120 | Aug., 1976 | Kessels | 378/80.
|
4115689 | Sep., 1978 | Won | 378/79.
|
5161179 | Nov., 1992 | Suzuki et al. | 378/161.
|
5181233 | Jan., 1993 | Rink et al. | 378/79.
|
Primary Examiner: Dzierzynski; Paul M.
Assistant Examiner: Nguyen; Kiet T.
Attorney, Agent or Firm: Austin; Reginald J.
Parent Case Text
FIELD OF INVENTION
This invention relates to a method and apparatus for use in performing
non-contact analytical evaluation of a semiconductor wafer and is a
continuation-in-part of patent application bearing Ser. No. 07/833,470 and
filed on Feb. 6, 1992, now abandoned.
Claims
What is claimed is:
1. An apparatus for use in performing non contact analytical evaluation of
a semiconductor wafer, comprising a sealable container having wall means
defining a chamber, which upon sealing the container is then isolated; the
sealable container also comprising:
window means into the sealable container, the window means comprising a
planar membrane composed solely of a substantially non-crystalline organic
polymer, the window means being capable of allowing a probe beam of
radiant energy of wavelengths corresponding to X-ray, visible light, or
ultra-violet light frequency bands, suitable for non contact analytical
evaluation of the semiconductor wafer to pass therethrough into the
isolated chamber while non-substantially affecting diffraction and
attentuation of the beam and which is capable of passing radiant energy
outwardly from the isolated chamber while non-substantially affecting the
diffraction and attenuation of the radiant energy;
releasable securing means to secure the window means to the wall means and
over an opening to the chamber and to enable removal of the window means
to allow for insertion of a wafer through the opening and into the
chamber;
sealing means to seal the window means to the wall means when the window
means is secured to the wall means; and
positioning means for positioning the semiconductor wafer within the
isolated chamber in a location aligned with the window means to allow the
probe beam of radiant energy to be directed through the window means at
the semiconductor wafer and to allow radiant energy to pass outwardly from
the semiconductor wafer within the chamber.
2. An apparatus according to claim 1, wherein the sealable container is
composed of a charge conducting material.
3. An apparatus according to claim 1 wherein the window means comprises a
single window, the window comprising a membrane having a surface area
which is larger than the area on the wafer surface which is to be tested.
4. An apparatus according to claim 1 wherein the window means comprises a
single window, the window comprising a membrane having a surface area
which is larger than the area on the wafer surface which is to be tested.
5. An apparatus according to claim 1 wherein the membrane is capable of
passing infra-red light.
6. A method of evaluating a semiconductor wafer by non contact analytical
evaluation, comprising the steps of:
in a first environment, inserting the semiconductor wafer into a chamber
within and defined by walls of an open container and positioning the
semiconductor wafer in a location within the chamber, the container
capable of being selected to isolate the chamber from an outside
environment, the container having window means into the chamber, the
window means comprising a planar membrane composed solely of a
substantially noncrystalline organic polymer, the window means being
capable of allowing a probe beam of radiant energy of wavelengths
corresponding to X-ray, visible light, or ultra-violet light frequency
bands, suitable for non contact analytical evaluation of the wafer, to
pass there through while non-substantially affecting diffraction and
attenuation of the beam, and which is capable of passing radiant energy
outwardly from the isolated chamber while non-substantially affecting
diffraction and attenuation of the radiant energy;
sealing the container to isolate the chamber and the semiconductor wafer
from the first environment with the semiconductor wafer aligned with the
window means to enable the probe beam of radiant energy to be directed
through the window means and at the semiconductor wafer;
moving the sealed container into a second environment and with the chamber
and semiconductor wafer isolated form the second environment;
passing the probe beam of radiant energy through the window means at the
semiconductor wafer during the performance of the non contact analytical
evaluation; and
analyzing radiant energy from the semiconductor wafer which passes
outwardly from the wafer within the chamber through the window means to
determine structural characteristics about the semiconductor wafer.
7. A method according to claim 6 wherein the window means comprises a
single window comprising a membrane having a surface area which is larger
than the area of the wafer surface to be tested to expose any part of the
wafer to the beam when directed towards the wafer at an angle of less than
normal to the wafer surface, the method comprising directing the beam
successively from localised position to localised position of the wafer
surface and analyzing radiant energy from the wafer at each localized
position.
8. An apparatus for use in performing non contact analytical evaluation of
a semiconductor wafer, comprising a sealable container having wall means
defining la chamber, which upon sealing the container is then isolated;
the sealable container also comprising:
window means into the sealable container, the window means comprising a
membrane of nitrocellulose polymer having a thickness of approximately 3.0
microns, the window means being thereby capable of allowing a probe beam
of radiant energy of wavelengths corresponding to X-ray, visible light, or
ultra-violent light frequency bands, suitable for non contact analytical
evaluation of the semiconductor wafer to pass therethrough into the
isolated chamber while non-substantially affecting diffraction and
attenuation of the beam and which is capable of passing radiant energy
outwardly from the isolated chamber while non-substantially affecting the
diffraction and attenuation of the radiant energy; and
positioning means for positioning the semiconductor wafer within the
isolated chamber in a location aligned with the window means to allow the
probe beam of radiant energy to be directed through the window means at
the semiconductor wafer and to allow radiant energy to pass outwardly from
the semiconductor wafer within the chamber.
9. An apparatus for use in performing non contact analytical evaluation of
a semiconductor wafer, comprising a sealable container having wall means
defining a chamber, which upon sealing the container is then isolated; the
sealable container also comprising:
window means into the sealable container, the window means comprising a
planar membrane composed solely of a substantially non-crystalline organic
polymer, the window means being capable of allowing a probe beam of
radiant energy suitable for non contact analytical evaluation of the
semiconductor wafer to pass therethrough into the isolated chamber while
non-substantially affecting diffraction and attenuation of the beam and
which is capable of passing radiant energy outwardly from the isolated
chamber while non-substantially affecting the diffraction and attenuation
of the radiant energy; and
positioning means for positioning the semiconductor wafer within the
isolated chamber in a location aligned with the window means to allow the
probe beam of radiant energy to be directed through the window means at
the semiconductor wafer and to allow radiant energy to pass outwardly from
the semiconductor wafer within the chamber.
Description
BACKGROUND OF THE INVENTION
It is a well-known concern of individuals involved in semiconductor
research and development that analytically evaluating semiconductor wafers
within the confines of a clean room facility is an expensive and time
consuming proposition. Test equipment occupies costly clean room floor
space and special dress requirements for process monitoring personnel is
both time consuming and inconvenient.
In greater detail, semiconductor wafers are manufactured in clean room
environments during which noncontact and non-destructive analytical
evaluations are performed to monitor process steps as required. Typically
these analytical tests are performed in the confines of clean rooms having
a class environment rating appropriate for the type of semiconductor
devices being manufactured. The analysis can be by electromagnetic
radiation such as X-rays or visible light. One typical example would be to
determine the characteristics of a wafer surface after the completion of a
particular process step. To prevent the contamination of a wafer, it must
at all times during the manufacturing and process monitoring cycles be
kept in a clean room environment.
Clean room facilities of the type required for modern semiconductor wafer
manufacturing have been built utilizing the height of three stories of a
building. Of these three stories, the clean room is typically the middle
story and is sandwiched between two floors which house expensive air
purification equipment. A portion of this expensive floor space is simply
required for storing equipment which is needed for monitoring process
steps and which obviously dictates the minimum floor space needed for the
manufacturing of semiconductor wafers.
When working within a clean room environment all personnel are required to
wear special clothing to help maintain the clean room environment. This
clothing must be donned before entering the clean environment and removed
after they leave. This is an inconvenient and time consuming process. In
particular the process monitoring personnel who are required only for
monitoring process steps must enter and exit the clean room environment
many times daily.
Analysis methods may require special equipment which cannot be modified or
adapted for use within clean room environments and therefore semiconductor
wafers which are removed from the clean room environment for evaluation
are typically thrown in the garbage once the evaluation is finished for
fear of contamination. Equipment that is compatible with clean room
environments is very expensive and even then must first be precleaned
before being placed within the clean room environment. As well all
equipment for use within the clean room environment must only use internal
lubricants which are compatible for use within a clean room facility.
SUMMARY OF INVENTION
The present invention provides a method and apparatus for use in performing
non-contact and non-destructive analytical evaluation of a semiconductor
wafer and which seeks to overcome or minimize the above problems.
One aspect of the invention provides an apparatus for use in performing
non-contact analytical evaluation of a semiconductor wafer, comprising a
sealable container defining a chamber, which upon sealing the container is
then isolated; the sealable container comprising: window means into the
sealable container, the window means comprising a membrane of a
substantially non-crystalline organic polymer, the window means being
capable of allowing a probe beam of radiant energy of a particular
wavelength suitable for non-contact analytical evaluation of the
semiconductor wafer to pass therethrough into the isolated chamber while
non-substantially affecting diffraction and attenuation of the beam and
which is capable of passing radiant energy outwardly from the isolated
chamber while non-substantially affecting the diffraction and attenuation
of the radiant energy; and positioning means for positioning the
semiconductor wafer within the isolated chamber in a location aligned with
the window means to allow the probe beam of radiant energy to be directed
through the window means at the semiconductor wafer and to allow radiant
energy to pass outwardly from the semiconductor wafer within the chamber.
In use, the invention overcomes the expense of locating equipment for
analytically evaluating semiconductor wafers within the confines of the
clean room environment because the semiconductor wafer, protected from
dust particle contamination is analytically evaluated outside the clean
room environment. Equipment for use in performing analytical evaluation no
longer needs to occupy costly clean room floor space and the precleaning
of the equipment is not required. Process monitoring personnel are also no
longer required to don and doff special suits and therefore a considerable
amount of time is saved.
The window means may for example be made from any of the following
polymers, i.e. poly-propylene, poly-isoprene, poly-vinyl chloride,
poly-vinilydene fluoride, poly-carbonate, poly-methyl methacrylate,
poly-ethylene, or nitro-cellulose. Attenuation is influenced by the
thickness of the membrane. The choice of any non-crystalline organic
polymer, including those mentioned in the last preceding sentence is
dependent upon the maximum attenuation requirement of the user and whether
a particular polymer may be made sufficiently thin to act as a membrane
while satisfying those requirements. All of the polymers mentioned above
are substantially transparent to X-rays and of the group; poly-carbonate,
poly-methyl methacrylate, poly-ethylene, and nitro-cellulose are also
transparent to ultra-violet, infra-red and visible light.
Preferably the window means comprises a membrane of nitro-cellulose polymer
having a thickness in the order of three microns and having a uniformity
of less than about .+-. 0.2 microns. A membrane having these
characteristics exhibits an attenuation level to X=rays (1-2 .ANG.) of
less than about 1%. An apparatus according to the invention and having a
preferred nitro-cellulose polymer membrane may be used for X-ray
diffraction evaluation, photo-luminescence evaluation and
photo-reflectance evaluation since it is substantially transparent to
visible, ultra-violet and infra-red light as well as X-ray energy.
Preferably the area of the membrane of the window means is substantially
larger than the surface area of the semiconductor wafer to be tested, by
an amount sufficient to permit scanning of the complete wafer surface by
test equipment having probe beams requiring predetermined incident angles
other than ninety degrees.
Another aspect of the invention provides a method of evaluating a
semiconductor wafer by non-contact analytical evaluation, comprising the
steps of: in a first environment, inserting the semiconductor wafer into a
chamber within an open container and positioning the semiconductor wafer
in a location within the chamber, the container capable of being sealed to
isolate the chamber from an outside environment, the container having
window means into the chamber, the window means comprising a membrane of a
substantially non-crystalline organic polymer, the window means being
capable of allowing a probe beam of radiant energy of a particular
wavelength suitable for non-contact analytical evaluation of the wafer, to
pass there through while non-substantially affecting diffraction and
attenuation of the beam, and which is capable of passing radiant energy
outwardly from the isolated chamber while non-substantially affecting
diffraction and attenuation of the radiant energy; sealing the container
to isolate the chamber and the semiconductor wafer from the first
environment with the semiconductor wafer aligned with the window means to
enable the probe beam of radiant energy to be directed through the window
means and at the semiconductor wafer; moving the sealed container into a
second environment and with the chamber and semiconductor wafer isolated
from the second environment; passing the probe beam of radiant energy
through the window means at the semiconductor wafer during the performance
of the non-contact analytical evaluation; and analyzing radiant energy
from the semiconductor wafer which passes outwardly from the wafer within
the chamber through the window means to determine characteristics about
the semiconductor wafer.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the invention will now be described, by way of example,
with reference to the accompanying drawings in which:
FIG. 1 is a plan view of an apparatus according to the embodiment;
FIG. 2 is a cross sectional view of the apparatus of FIG. 1 and taken along
line II--II in FIG. 1; and
FIG. 3 is a view similar to FIG. 2, but to a larger scale, of the apparatus
in use.
DETAILED DESCRIPTION
In the embodiment, as shown in FIGS. 1, 2 an apparatus 10 for use in
performing non-contact analytical evaluation of a semiconductor wafer,
comprises a machined aluminum base 14, in the form of a cylinder, the
cylinder having a closed bottom end 17 and an open top end. The container
includes window means in the form of a planar window 11 which may be
releasably secured to the open end of the aluminum base 14 as shown in
FIGS. 1 and 2 with the use of screw threaded means in the form of screws
15 to form an isolated chamber 19. Securing of the window 11 is effected
by sandwiching continuous edge regions of the window between two
concentric rigid rings 12a, 12b with the screws 15 received in threaded
bores in the base 14. The chamber 19 is rendered airtight by a seal 16 in
the form of an `O` ring which is compressed between the bottom ring 12b
and the open end of the aluminum base 14. A positioning means is included
for positioning a semiconductor wafer. This is formed by a recessed cavity
13 in the cylinder end 17, the cavity diameter and location suitable for
positioning the wafer in a predetermined position relative to the window
11.
For reasons of practicality a membrane material for the window 11 would be
one that can be used in conjunction with a majority of the evaluations
mentioned. A membrane to be used in conjunction with X-rays of a
wavelength commonly used in X-ray diffraction analysis (1.54 .ANG.) must
be substantially non crystalline. The requirement that the membrane for
the window 11 be non crystalline arises because X-rays can diffract as
they pass through a medium having a crystal structure. A resulting
detected diffraction pattern would then not be due solely to the wafer 20
but to a combination of the membrane and the wafer 20 under test. This
alteration of the diffraction pattern due to the membrane would result in
inaccurate conclusions about the wafer's composition.
A membrane for use with the invention is manufactured by Micro Lithography
Incorporated of Sunnyvale California having a model number, PE 02-102-001.
The membrane is composed of a nitrocellulose polymer having an absolute
thickness of 2.85 microns and a uniformity of .+-. 0.2 microns. The
membranes are available in various diameters. The membranes are
inexpensive and are replaceable should one on an apparatus ever be
punctured while being handled.
The base 14 of the embodiment is a charge conducting material. Any charge
conducting material will suffice. Semiconductor wafers can be damaged by
electric discharges between surfaces which are at different voltage
potentials. A charge conducting base 14 for example would allow a
technician wearing a ground strap to touch the apparatus 10 thus
eliminating any potential difference between the container 10 and the
technician.
An apparatus 10 having a membrane window 11 composed of for example a nitro
cellulose polymer can be successfully used in conjunction with analytical
equipment for X-ray diffraction analysis, photoluminescence, and
photoreflectence testing of semiconductor wafers. The use of the apparatus
can possibly be extended to include X-ray topography and reflectomerry
analysis equipment. Modifications as to height of the window 11 from the
wafer 20 surface and to the exposed area of the window 11 relative to the
area of the wafer 20 may need to be considered for these uses.
In order to evaluate the wafer 20, in a clean room environment the wafer 20
is placed inside the apparatus 10 by locating the wafer 20 in the recessed
cavity 13 of the bottom end 17 of the base 14. The window 11 is then
positioned over the open end of the base 14 and the window 11 is secured
airtightly in position by the use of the rings 12a, 12b and fastening
screws 15 which compresses the `O` ring seal 16 thus isolating the chamber
19 which isolates the wafer 20 from an environment outside the apparatus.
The apparatus 10 is then removed from the clean room facility to allow the
semiconductor wafer 20 to be evaluated using non-contact analytical test
equipment located in a less clean environment.
The apparatus 10 containing the wafer 20 is placed on a test instrument
platform 32 to be used, as shown by FIG. 3, in conjunction with an
analytical test instrument. The test instrument is an X-ray diffractometer
and for clarity only the essential elements of the test instrument are
shown. The test instrument comprises a transmitter 30 or source of an
incident radiant energy beam 40 in conjunction with a receiver 31 for
detecting returned energy 50 from a semiconductor wafer 20 located in the
desired position in the cavity 13 .so as to be aligned with the window 11.
The wafer 20 upper surface is then systematically irradiated from
localized position to localized position with the beam 40 of incident
radiant energy from the transmitter 30. Analysis of the returned energy 50
from the wafer identifies structure of the wafer 20 and hence it's
composition. After analysis the apparatus 10 containing the wafer is
returned to the clean room environment, the wafer then being removed from
the apparatus for further processing.
At many stages during the manufacturing of semiconductor wafers it is
desirable to conduct various analytical evaluations on the wafers to
monitor manufacturing process steps. Typical evaluations for example may
be by X-ray diffraction analysis, photoluminescence analysis or
photoreflectence analysis of the semiconductor wafer. Each of the
non-contact evaluations mentioned requires a probe beam of appropriate
radiant energy to be incident on the surface of the wafer and the returned
energy from the wafer 20 to be analyzed. Diffraction analysis requires a
probe beam of X-rays while photoluminescence and photoreflectence testing
both require probe beams having radiant energy in the visible light
spectrum.
Photoluminescence testing, probes a semiconductor wafer with visible or
ultraviolet light. The emitted radiation is usually in the visible or
infrared portion of the electromagnetic spectrum. A window 11 to be used
in photoluminescence analysis must then consist of a membrane which is
substantially transparent to radiant energy within these three ranges of
the electromagnetic spectrum. o Photoreflectence testing on the other hand
uses a probe beam in the visible light spectrum and analyses returned
energy also in the visible region of the spectrum.
The attenuation of an X-ray probe beam of a wavelength commonly used in
diffraction analysis (1.54 .ANG.) becomes negligible with a
nitro-cellulose polymer membrane having an absolute thickness of about 3
microns and a uniformity of less than about .+-. 0.2 microns. Conveniently
this same membrane is also substantially transparent to visible and
ultraviolet light and only moderately attenuates infrared radiation thus
allowing an apparatus having a nitrocellulose polymer membrane to be used
for more than one type of evaluation.
Non-contact testing of the above types inherently requires probe beams to
be incident on the wafer surface at predetermined angles of incidence. For
diffraction testing for example it is typical for the probe beam to be
incident to the surface of the wafer at an angle of around thirty degrees.
Thus for semiconductor wafer testing using the above mentioned tests, the
surface area of the window means must be larger than the surface area of
the wafer for accessibility by the probe beam if the whole of the wafer
surface is to be tested in successive stages.
In modifications (not shown) of the embodiment, window means are included
of materials different from that of the embodiment; such materials are
mentioned hereunder. However, in each case, a membrane of the window means
is of a substantially non-crystalline organic polymer which is
satisfactory for use with X-rays.
X-ray diffraction analysis restricts a membrane composition to consist of
compounds formed from elements with low atomic numbers. Electrons within
electron clouds, which surround atoms are known to have an attenuating
effect on X-rays. Electron clouds surrounding atoms of elements having a
low atomic number have fewer electrons within their clouds than atoms of
elements having higher atomic numbers and hence attenuate X-rays to a
lesser degree. Membranes having substantially non crystalline organic
polymer membranes tend to be transparent to X-rays and thus may be used
satisfactorily in this application.
Absolute thickness of any membrane is a limiting factor for analytical
methods using X-rays, infrared or i visible light. The thicker the
membrane the greater is the attenuation of both the incident 40 energy and
the returned radiant energy 50 from the wafer 20. Bearing in mind this
consideration, a variety of polymer materials may be used with attenuation
in each case dependent upon the atomic weights of included elements and
the practical minimum thickness of the membranes. These materials include
poly-propylene, poly-isoprene, poly-vinyl chloride, poly-vinilydene
fluoride, poly-carbonate, poly-methyl methacrylate, poly-ethylene, or
nitro-cellulose.
The variation in uniformity of thickness across a surface of a membrane can
contribute to non-uniform attenuation of incident energy 40 and the
returned energy 50. This difficulty can be overcome by first calibrating
the analytical test instrument to compensate for previously measured
attenuation variances over a membrane's surface using a highly uniform
membrane conveniently alleviates much of the effort required to
characterize individual membrane attenuation variances and much of the
test instrument calibration effort.
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